The invention is based on a priority application EP 04 360 063.4 which is hereby incorporated by reference.
The present invention relates to the field of coupling of radio frequency (RF) signals.
In the framework of telecommunication systems providing RF based wireless signal transmission, a plurality of signal modulation techniques is widely applied. Typically, signal modulation and signal amplification is realized by two separate components of an RF circuit. Therefore, a modulated RF signal generated by an RF modulator has to be transmitted to an amplifier for providing sufficient amplification for signal broadcasting.
For transmission of RF signals between any two units of a RF circuit, differential signal transmission provides an effective means for reducing noise level and to reject common-mode voltages. Differential signal transmission is typically realized by means of two separate transmission lines for separately transmitting of two, typically phase shifted, radio frequency (RF) signals forming the differential signal. Differential signal transmission is preferably applicable in combination with two-phase modulation techniques, such as In-Phase Quadrature (IQ), Quadrature Phase-Shift Keying (QPSK), Binary Phase-Shift Keying (BPSK), Differential Phase-Shift Keying (DPSK), Quadrature Amplitude Modulation (QAM) and Pulse Code Modulation (PCM).
For many applications a modulated RF signal has to be measured in order to determine the quality of the modulation and to detect potential signal distortions that may arise due to improperly tuned modulation parameters. Therefore, at least a part of a generated modulated RF signal has to be coupled to a corresponding detector. Coupling of the modulated RF signal, i.e. deriving of at least a part of the RF signal and providing the derived part to a detector, has to meet stringent requirements in order to prevent signal distortion. Typically, coupling of a differential RF signal refers to coupling to anyone of the two separate RF signals. Hence, coupling is applied to one of the two separate transmission lines connecting two RF units, that may be implemented as e.g. a modulator and a power amplifier.
In general, coupling of an RF signal can be effectively realized by parallel arrangement of an additional transmission line located at a specific distance from the main transmission line connecting the two RF units. Typically, this distance is specified by the wavelength of the RF signals and equals to λ/4. Even for the RF bands operating in the GHz range, this distance may be as large as several cm. This requires a rather spacious implementation of a coupling mechanism and does not allow a compact and cost-efficient design of a coupling arrangement.
Alternative coupling arrangements might be implemented by making use of directly connecting electronic components like resistors and capacitors to the RF transmission line. However, this type of coupling arrangements may have a serious impact on the transmission of the radio frequency signal provided by the respective transmission line.
The present invention therefore aims to provide a space-saving coupling arrangement for differential transmission lines featuring a minimal impact on the transmission of the differential RF signal.